Abstract
Abstract. Atmospheric composition and chemistry above tropical rainforests is currently not well established, particularly for south-east Asia. In order to examine our understanding of chemical processes in this region, the performance of a box model of atmospheric boundary layer chemistry is tested against measurements made at the top of the rainforest canopy near Danum Valley, Malaysian Borneo. Multi-variate optimisation against ambient concentration measurements was used to estimate average canopy-scale emissions for isoprene, total monoterpenes and nitric oxide. The excellent agreement between estimated values and measured fluxes of isoprene and total monoterpenes provides confidence in the overall modelling strategy, and suggests that this method may be applied where measured fluxes are not available, assuming that the local chemistry and mixing are adequately understood. The largest contributors to the optimisation cost function at the point of best-fit are OH (29%), NO (22%) and total peroxy radicals (27%). Several factors affect the modelled VOC chemistry. In particular concentrations of methacrolein (MACR) and methyl-vinyl ketone (MVK) are substantially overestimated, and the hydroxyl radical (OH) concentration is substantially underestimated; as has been seen before in tropical rainforest studies. It is shown that inclusion of dry deposition of MACR and MVK and wet deposition of species with high Henry's Law values substantially improves the fit of these oxidised species, whilst also substantially decreasing the OH sink. Increasing OH production arbitrarily, through a simple OH recycling mechanism , adversely affects the model fit for volatile organic compounds (VOCs). Given the constraints on isoprene flux provided by measurements, a substantial decrease in the rate of reaction of VOCs with OH is the only remaining option to explain the measurement/model discrepancy for OH. A reduction in the isoprene+OH rate constant of 50%, in conjunction with increased deposition of intermediates and some modest OH recycling, is able to produce both isoprene and OH concentrations within error of those measured. Whilst we cannot rule out an important role for missing chemistry, particularly in areas of higher isoprene flux, this study demonstrates that the inadequacies apparent in box and global model studies of tropical VOC chemistry may be more strongly influenced by representation of detailed physical and micrometeorological effects than errors in the chemical scheme.
Highlights
Global emissions of non-methane biogenic volatile organic compounds (BVOCs) are estimated to total 1150 Tg C yr−1 (Guenther et al, 1995) and exceed those of their anthropogenic counterparts by a factor of 10 (WMO, 1995)
Whilst we cannot rule out an important role for missing chemistry, in areas of higher isoprene flux, this study demonstrates that the inadequacies apparent in box and global model studies of tropical VOC chemistry may be more strongly influenced by representation of detailed physical and micrometeorological effects than errors in the chemical scheme
Convection of BVOC oxidation products is a major source of HOx in the upper atmosphere (Poisson et al, 2000; Tan et al, 2001), whilst relatively long-lived oxides of nitrogen such as PAN can influence remote tropospheric NOx, and ozone chemistry
Summary
Global emissions of non-methane biogenic volatile organic compounds (BVOCs) are estimated to total 1150 Tg C yr−1 (Guenther et al, 1995) and exceed those of their anthropogenic counterparts by a factor of 10 (WMO, 1995). The best fit is located at the furthest-right point of this dark blue area (black dot in Fig. 1), this quite broad general minima indicates that monoterpene emissions can be traded on an approximately 1:1 basis for isoprene emissions for only a small penalty in fit This is not altogether surprising given the broadly comparable atmospheric chemistry and lifetimes of these species, at least as far as the model is concerned. To test the impact of higher NO fluxes, the model was run with the measured soil NOx emissions, using a canopy reduction of 75%. This resulted in a 400% overestimation of midday [NO2] and 180% overestimation of [NO] compared to the measurements. Further discussion of this issue is beyond the scope of this paper, but will be addressed in Whalley et al (2010a)
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